This invention relates to lifting devices or slings for lifting helicopter blades when installing a blade in a central hub to complete a helicopter rotor system.
Helicopters, generally, have two to six blades attached to a central hub which in turn is attached to the drive shaft of the engine. The cross section of a helicopter blade is shaped like a cross section of an airplane wing; the blade is an air foil and produces lift. The blades and hub make up a rotory wing, or rotor. On older helicopters the blades were attached to the hub by horizontal hinges which allowed them to "flap", or move up and down, and they were also attached by vertical hinges which allowed them to "lead and lag", or move forward and backward, relative to a straight-out position. Since the blade is a lifting surface, means are provided to change the angle of attack or pitch of the blade by rotating the blade along its longitudinal axis. Current helicopters provide a mechanical coupling between the rotor blades and the hub by terminating the inboard end of a blade in a flexure portion which accommodates the necessary movement of the blade by deflection. Flexure type couplings not only eliminate the two hinges, but the blade incorporates a pretwist at the blade root to minimize torsional stresses. This pretwist is in the range of 30.degree.. Also, the blade is typically installed at some predetermined flap angle other than horizontal.
Typically, the blade is joined to the flexure through some type of clevis joint which may be a double clevis with a common middle leg. For a better understanding of the typical flexure couplings and the blade terminations, see U.S. Pat. No. 4,650,401. Since the rotor-blade-to-flexure coupling is a highly stressed connection, the fit up tolerances are snug, and installation and removal of the blades requires accurate alignment between the blades and the hub or flexure.
In the prior art, a pair of conventional fabric slings with a spreader bar between them was used in conjunction with an overhead hoist to support the blade during removal and installation and to provide the necessary freedom of movement to align the two mating parts. As previously discussed, aligning the blades requires simultaneous flap and pitch movement which was provided by a man applying restraining moments on the blade to maintain the flapped and pitch position. Of course, shifting the center of gravity of the blade caused a necessary correction in the other alignment parameters.
It is an object of this invention to provide a rotor blade sling which in combination with an overhead hoist accommodates angular adjustment about the conventional X, Y and Z axes as well as translational displacement about these axes. It is a further object of this invention to provide a device which accommodates angular adjustment about one axis with minimum effect on the other axes by eliminating unbalanced moments; i.e., rotating about one axis produces only the desired change in orthographic projection of the points on the blade.